1. Field of the Invention
This invention relates to a wavelength variation measuring apparatus for measuring any variation in an oscillated wavelength, for example, from a laser source, and particularly to a wavelength variation measuring apparatus suitable for application to an interference apparatus for measuring physical amounts such as the length, shape, speed and refractive index of an object to be measured with the wavelength of laser light as a reference.
2. Related Background Art
A light heterodyne interference apparatus is known as an interference apparatus capable of detecting the length shape, etc. of an optical member such as a lens or a mirror relatively highly accurately by the utilization of light wave interference.
FIG. 1 of the accompanying drawings is a schematic view of the essential portions of a light heterodyne interference apparatus according to the prior art which utilizes a laser source and an AD modulator.
In FIG. 1, laser light (frequency f.sub.0) emitted from a laser source 51 is divided into two light beams by a polarizing beam splitter 52. The frequency shift of an incident light beam is effected by AO modulators 53 and 54 disposed in the optical paths of the divided two light beams. Thereby there are obtained two light beams of frequency f.sub.0 +f.sub.1 and frequency f.sub.0 +f.sub.2, respectively. The two light beams subjected to the frequency shifting are synthesized by a polarizing beam splitter 56, whereby the combined light beam is directed as a light beam for light heterodyne measurement including two light beams slightly differing in frequency to an object to be measured through a mirror 55.
Also, as light wave interference, the fluctuation of an oscillated wavelength from light source means greatly affects measurement accuracy. It is therefore important to achieve the stabilization of the oscillated wavelength.
FIG. 2 of the accompanying drawings is a block diagram of the essential portions of an interference apparatus which is proposed in U.S. Pat. No. 4,907,886 wherein the stabilization of the oscillated wavelength of laser light from a laser source is achieved.
In FIG. 2, laser light from a laser source 70 is divided into two light beams by a light divider 73. The two light beams are caused to enter two interferometers 71 and 72 disposed in parallel. With one interferometer 72 as a reference interferometer, a variation in the phase of an output from a phase portion 75 is fed back to the laser source 70. Thereby the oscillated wavelength from the laser source 70 is stabilized and the wavelength of the laser light entering the other interferometer 71 is stabilized. However, in the light heterodyne interference apparatus of FIG. 1, it is a premise that the oscillated wavelength of the laser source itself is stable. It has therefore been necessary to endow the laser source itself with the wavelength stabilizing function in precise measurement using the wavelength of laser light as a reference.
Particularly where a semiconductor laser is used as a laser source, it has been necessary to effect control by the use of a highly accurate temperature control device or an expensive optical element such as an etalon. This has led to the problem that the light source unit becomes bulky and complicated.
Also, in the interference apparatus shown in FIG. 2, a reference interferometer exclusively for use for stabilizing the oscillated wavelength from the laser source is required, and this also has led to the problem that the entire apparatus becomes bulky and complicated.